1. Field of the Invention
The invention relates to an exchange for computer-controlled switching of a plurality of lines each of which carry digital time-interleaved data, via a switching matrix, which transmits the data in parallel in the form of data words comprising a plurality of data bits.
2. Description of the Related Art
Such exchanges in which more specifically the switching matrices have different structures, are generally known and are used for switching a large number of subscriber stations. As a fault-dependent failure of such an exchange affects a large number of subscribers, the reliability of exchanges of this type must satisfy high requirements, particularly the computers for producing the control data for setting the switching matrices, since a fault in this control computer may cause the total overall exchange to fail. It is consequently customary to use in switching exchanges at least two or three identical control computers, as is described, for example, in the periodical "Phillips Technical Review", Vol. 41, No. 1983/84, No. 1, pages 1 to 11. This article describes also, how, by using a special fault-tolerant concept it can be obtained that one of a plurality of computers may fail without causing breakdown of the total switching exchange.
However in all the concepts no fault protection of the actual switching matrices is used, as it is assumed that, for example a faulty component only affects a particular switched path, that is to say the two participating subscribers, it being possible in certain switching matrix concepts to switch an existing connection, so that the faulty place is bypassed, when the fault has been determined. To determine a fault in a newly set-up connection it is customary to transmit pre-defined test data via the link before the data to be transmitted and to check at the end of the link whether these test data are correctly received. Such a method is however not only time-consuming but in many types of switching matrices, or when there are many faulty positions it is not possible to establish a path bypassing the faulty place, and, in addition, faults occurring during the period of time a switched path exists cannot be detected without further measures. Particularly during the transmission of data this may have very serious consequences.
It is indeed known to protect the data to be transmitted by adding redundant bits thereto, which may be realized in various ways, or by a complete encoding, but then, depending on the quantity of redundant bits added to the data, a more or less complicated reconstruction of mutilated data words and at any rate a complicated recognition of mutilated data words which can not be restored again may be the result. However, when an error occurs and is detected it is extremely difficult to repair it since a switching exchange must never be switched off.